As a current-type light-emitting device, Organic Light-Emitting Diodes (OLED) have been widely used in display devices with high performance. A traditional Passive Matrix OLED requires a shorter driving time for a single pixel as a display size increases, therefore a transient current should be increased and power consumption increases. Also, an application of a great current may lead to an over-large voltage drop on lines of nanometer Indium Tin Oxides (ITO), and cause an over-high operation voltage of the OLED, which may in turn decrease its efficiency. As compared, an Active Matrix OLED (AMOLED) may settle these problems perfectively by scanning input OLED currents progressively by means of switching transistors.
In a design for an array substrate of the AMOLED, a major problem needed to be settled is a non-uniformity in brightness among pixel unit circuits.
Firstly, the AMOLED constructs the pixel unit circuit with Thin-Film Transistors (TFTs) to provide a corresponding current to the OLED device. In the prior art, Low Temperature Poly-Silion TFTs (LTPS TFTs) or Oxide TFTs are generally used. As compared with a general amorphous-Si TFT, the LTPS TFT and the Oxide TFT have a higher mobility and a better stability, and is more suitable to be applied to the AMOLED display. However, because of the limitation of the crystallization process, there is a disadvantage of non-uniformity in electric parameters such as threshold voltages, the mobility and the like while manufacturing LTPS TFTs on a glass substrate with a large area. Such non-uniformity may be transformed as a current difference and a brightness difference among the OLED display devices, and be perceived by viewer, which is called as a Mura phenomenon. The Oxide TFT has a good uniformity in the process, but similar to the a-Si TFT, the threshold voltage of the Oxide TFT would drift when a voltage is applied for a long time and under a high temperature. Amounts of the drift in the thresholds of the TFTs in respective parts on a panel would be different because displayed contents are different, which may lead to difference in the display brightness. Because such difference relates to an image displayed previously, it is generally shown as an image sticking phenomenon.
Secondly, in the display application with a large size, a power supply voltage at a region close to a supply position of an ARVDD power supply is higher as compared with that at a region far away from the power position in the array substrate, because power lines on the array substrate have certain resistances and the driving current for all pixels are provided by the power supply (ARVDD), and such phenomenon is called as power supply drop (IR Drop). The IR Drop may also lead to the current differences among the different regions and in turn generate the Mura phenomenon as display, since the voltage of the ARVDD power supply is associated with the current. The LTPS process constructing the pixel unit with P-Type TFTs is sensitive to this problem especially, because its storage capacitor is connected between the ARVDD and gates of the driving transistors TFTs, and a gate-source voltage Vgs of the driving transistor TFT would be affected directly when the voltage of the ARVDD changes.
Thirdly, the OLED device may also cause the non-uniformity in the electric performance because of a non-uniformity in thicknesses of a mask during an evaporation process. For the a-Si or Oxide TFT process constructing the pixel unit with N-Type TFTs, its storage capacitor is connected between a gate of a driving transistor TFT and an anode of the OLED, and the gate-source voltages Vgs applied to the driving transistors TFT would be different actually if the voltages at the anodes of the respective OLEDs are different when a data voltage is transferred to the gates of the respective driving transistors TFTs, such that the different driving currents may cause the difference in the display brightness.
The AMOLED may be divided into three categories based on the driving types: a digital type, a current type and a voltage type. The digital type driving method may implement gray scales by a manner of controlling driving timing with the TFTs as switches without compensating the non-uniformity, but its operation frequency would increase doubled and redoubled as the display size grows, which leads to a great power consumption, and reach a physical limitation of the design within a certain range, therefore it is not suitable for the display application with the large size. The current type driving method may implement the gray scales by a manner of providing the driving transistors TFTs with currents having different values directly, and may compensate the non-uniformity of the driving transistors TFTs and the IR drop better, but when a signal having a low gray scale is written, a over-long writing time may be raised because a small current charges a big parasitic capacitor on a data line. Such problem is especially serious and even can not be overcome in the display with the large size. The voltage type driving method is similar to a driving method for the traditional Active Matrix Liquid Crystal Display (AMLCD) and provides a voltage signal representing the gray scale by a driving IC, and the voltage signal may be transformed to a current signal for the driving transistors inside the pixel circuit so as to drive the OLED to realize the luminance gray scales. Such method has advantages of a quick driving speed and simple implementation, which is suitable for driving the panel with the large size and widely used in industry, however it needs to design additional TFTs and capacitor devices to compensate the non-uniformity among the driving transistors TFTs, the IR Drop and the non-uniformity of OLEDs.
FIG. 1 illustrates a pixel unit circuit in the prior art. As illustrated in FIG. 1, the pixel unit circuit comprises two thin film transistors T2 and T1, and one capacitor C. The pixel unit circuit illustrated in FIG. 1 is a typical structure for a pixel circuit of a voltage driving type (2T1C). Wherein the thin film transistor T2 operates as a switching transistor, transfers a voltage on a data line to a gate of the thin film transistor T1, which operates as a driving transistor, and the driving transistor transforms the data voltage to a corresponding current to be supplied to an OLED device. The driving transistor T1 should be in a saturation zone when it operates normally, and provide a constant current during a scanning period for one row. The current may be expressed as follows:
      I    OLED    =            1      2        ⁢                  μ        n            ·              C        OX            ·              W        L            ·                                    (                                          V                data                            -                              V                OLED                            -                              V                thn                                      )                    2                .            
Wherein μn is a mobility of carriers, COX is a capacitance value of a capacitor in an oxide layer at the gate,
  W  Lis a width-length ratio of the transistor, Vdata is a signal voltage on the data line, VOLED is an operation voltage of the OLED, Vthn is a threshold voltage of the driving transistor TFT, which is a positive value for an enhanced TFT and is a negative value for a depletion TFT. It can be seen from the above equation that the currents would be different if the Vthn is different among the different pixel units. If the Vthn of the driving transistor TFT in a pixel unit drifts as time elapses, the currents before and after drifting would be different and the image sticking may occur. Also, the difference in the current may be also caused by difference in the operation voltages of the OLEDs because of a non-uniformity in the OLED devices.
There are many pixel structures for compensating the non-uniformity of the Vthn, the drift of the Vthn and the non-uniformity of the OLEDs, and they may be divided into two classes, an internal compensation and an external compensation, generally. The internal compensation is a compensation manner for, inside a pixel, storing information on the threshold voltage of the driving transistor TFT in the pixel with TFTs and a capacitor and feeding back the same to a bias voltage Vgs of the driving transistor TFT, and FIG. 2a is a pixel unit circuit constituted by enhanced TFTs with the internal compensation manner in the prior art, while FIG. 2b is a pixel unit circuit constituted by depletion TFTs with the internal compensation manner in the prior art. As illustrated in FIGS. 2a and 2b, the pixel unit circuit with the internal compensation manner in the prior art comprises a driving transistor, which is a thin film transistor, a gate and a source of the driving transistor are connected with each other, a drain of the driving transistor is connected with an anode of an OLED, and a cathode of the OLED is connected with a second power supply voltage ELVSS. Such structure is only applicable to the enhanced TFT, but for the depletion TFT, the TFT is still turned on when a voltage at the gate of the TFT is 0, therefore the voltage stored through the TFT would not include any information on the Vthn such that the non-uniformity in the Vthn can not be compensated.
Another compensation manner is the external compensation, that is, its compensation manner is as follows: I-V characteristics of the driving transistor and I-V characteristics of the light-emitting device are read to an external sensing circuit by TFTs inside the pixel, driving voltage value required to be compensated is calculated and fed back to a chip in a driving panel. FIG. 3 is a pixel unit circuit with the external compensation manner in the prior art. As illustrated in FIG. 3, the pixel unit circuit with the external compensation manner in the prior art comprises: an Active Matrix Organic Light-Emitting Diode (AMOLED), a display row selector, a sensor row selector, a column readout, an image processing LSI, an Analog-Digital Convertor (ADC) and an ASIC Processor (AP). Wherein the ASIC Processor (AP) provides display data to the image processing LSI, the AMOLED comprises an array of pixel unit circuits and reads out the currents or voltages of the respective pixel unit circuits by the column readout. As illustrated in FIG. 3, a triangle frame between the column readout and the ADC represents an amplifying and compensating circuit. Given a data voltage as a reference voltage, when the voltage flowing out from the column readout is smaller than the reference voltage, it indicates that the voltage of the pixel unit circuit at this position is needed to be compensated, and the voltage from the column readout is compensated by the amplifying and compensating circuit, so that the voltage or current of the driving transistor and/or the OLED device in the corresponding pixel unit circuit may be compensated.
The internal compensation and the external compensation have their own advantages and disadvantages. Generally, the internal compensation may only compensate the non-uniformity and the drifts of the threshold voltages of the driving transistor TFTs under limitations of a limited space and a circuit structure, while the external compensation may compensate the non-uniformity in the threshold voltages and the non-uniformity in the mobility of the driving transistor TFTs, and may also compensate some nonideal factors such as an ageing of the OLED, by implementing complex algorithm by means of the external integrated circuit chip(s). However, a compensation range of the external compensation is limited, its compensating voltage can not exceed a maximum range for voltage on the data line (DATA), while an internal driving voltage obtained by the internal compensation circuit may exceed the maximum range for the voltage on the data line. If the internal compensation and the external compensation may be combined with each other, their advantages may be acquired together.